Surgical procedures have been greatly assisted by the implementation of navigation systems. Navigation systems assist in surgery by providing previously acquired imaging information, such as magnetic resonance imaging (MRI), during surgery to visualize tissue morphology and locate target areas. Navigation systems may also be used to track surgical instruments and their location within the tissue during surgery, typically incorporating information from previously acquired imaging data. As an example, minimally invasive brain surgery may incorporate navigation systems to map a target area for surgical resection and access the target area with minimal damage to healthy brain tissue.
Surgical procedures that exert pressure on tissues and organs or alter their composition may produce deformation of tissue. For example, deformation of brain tissue may occur when a craniotomy is opened and pressure on the brain is relieved, when a surgical device such as a surgical port or catheter is introduced into the brain, or when tissue is removed during surgery such as in a tumour resection. The tissue deformation may render the surgical plan based on pre-operative imaging inaccurate and reduce the usefulness of the image-guided therapy. Deformation of tissue and its effects on the accuracy and precision of surgical procedures is an ongoing area of investigation and research, and there is a need for effective means to detect such deformation for surgical planning, navigation, and analysis. While much of the following discussion relates to surgical procedures in the brain as examples, similar issues arise in surgery to the spine and other orthopedic applications and the techniques are generally applicable.
The complexities associated with tissue shifts that occur during surgery are not well addressed by currently available systems and methods. For example during a craniotomy, when a large portion of the skull of a patient is removed to allow for access to the brain, the brain tends to swell outside of the remaining skull that is encasing the brain due to a pressure differential between the brain and the operating room. This brain swelling, and brain sag due to gravity, may lead to a significant shift in the brain tissue, often on the order of 1-2 cm. Additionally, as a tumor is resected from the brain, the position of the remaining tissue may shift relative to the pre-operative images as a result of the decreased volume. These mechanisms of brain swelling, sag, and shift may result in significant variations between pre-operative and intra-operative brain positions.
Thus, there is a need for effective means to detect tissue deformation resulting from various causes including tissue resection, swelling, and surgical tool insertions, to accommodate those changes and to allow for improved surgical planning, navigation, and analysis.